HF is generally recognized as the frequency range 2-30 MHZ. For the purposes of this specification the term is intended to cover any frequency up to 30 MHZ and may include frequencies above 30 MHZ. The term is intended to designate a type of radio transmission usually involving ionospheric reflection, from the F-layer, the maximum usable frequency usually being somewhat less than 30 MHZ.
Commercial and military systems are known employing HF transmission between a large number of ground stations. For example, an army may possess portable transceivers carried on vehicles or as "man-packs" which rely on HF data transmission. Because of the nature of HF transmissions, usually involving ionospheric reflection, there is a high probability of errors in data transmission caused by bursts, rapid fading and gaussian noise. Further since a military system will be required to operate in all conditions and any geographic location, e.g. the Arctic circle, where disturbed ionospheric conditions of propagation may exist, these causes of transmission error may be compounded.